Slow wave structure having oppositely curved conductors disposed about the beam and mounted transversely between opposite walls



NOV. 14, 1967 J FROQM 3,353,058

SLOW WAVE STRUCTURE HAVING OPPOSITELY CURVED CONDUCTORS DISPOSED ABOUT THE BEAM AND MOUNTED TRANSVERSELY BETWEEN OPPOSITE WALLS Filed July 9, 1964 3 Sheets-Sheet 1 Inventor d0 (6 YN FROG/1 Nov. 14, 1967 FROOM 3,353,058

SLOW WAVE STRUCTURE HAVING OPPOSITELY CURVED CONDUCTORS 4 DISPOSED ABOUT THE BEAM AND MOUNTED, TRANSVERSELY BETWEEN OPPOSITE WALLS Filed July 9, 1964 5 Sheets-Sheet 2 Inventor JOCELYN FROOM 3,353,058 UCIORS NOV. 14, 1967 FROQM SLOW WAVE STRUCTURE HAVING OPPOSITELY CURVED com) DISPOSED ABOUT THE BEAM AND MOUNTED TRANSVERSELY BETWEEN OPPOSITE WALLS 5 Sheets-Sheet 5 Filed July 9, 1964 lnvenlor \JOCELYN FkOOM United States Patent 3,353,058 SLOW WAVE dTRUCTURE HAVING UPPGSITELY IURVED CQNDUCTGRS DISPOSED ABGUT THE BEAM AND MOUNTED TRANSVERSELY RE- TWEEN ()PlfldiTE WALLS Jocelyn Froom, llishops Stortford, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Filed July 0, 1964. Ser. No. 381,334 Claims priority, application Great Britain, Aug. 16, 1963, 32,475/63 4 (Ilaims. (Cl. 315-35) The invention relates to travelling wave tubes and is particularly concerned with slow wave structures for use in such tubes.

In accordance with the present invention there is provided a travelling wave tube having a cylindrical electron beam and a slow wave structure whose beam-interacting portions conform closely to the profile of the beam and provide a tunnel therefor, wherein the said beam-interacting portions of the slow wave structure include a pair of similarly slotted but oppositely curved metal sheets having fiat longitudinally extending marginal portions on each side fixed to metal side walls of the structure and wherein each sheet is slotted to form with the side walls a stub-supported meander line providing a zigzag conducting path from end to end of the slow wave structure between the members of a set of transverse slots each of which extends from one of the side walls, over the curved portion of the sheet beyond the axis of the tunnel, and terminates either substantially short of or substantially beyond the junction of the curved portion of the sheet with the opposite fiat marginal portion.

Embodiments of the invention will be described with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic cross sectional view of a slow wave structure having two stub-supported meander lines for interaction with an electron beam flowing between them;

FIG. 2 is a diagrammatic plan view of a portion of a metal sheet forming one of the meander lines of FIG. 1;

FIG. 3 is a modification of the arrangement of FIG. 1 to accommodate a cylindrical electron beam; and

FIGS. 4 and 5 are diagrammatic perspective views of parts of travelling wave tubes according to the invention cut away to show respective differently dimensioned slow wave structures.

In FIG. 1 a slow wave structure is provided by a pair of stub supported meander lines joined between parallel opposing side walls 2 and 3. The side walls are subdivided into portions 2a, 2b and 2c, 3a, 3b and 3c, the first meander line being in the form of a slotted metal sheet 1 clamped between the side wall portions b and c, the second meander line being an identical metal sheet In sandwiched between the wall portions 0 and b. A ribbon-shaped electron beam 9 is represented as flowing between the sheets 1 and 1a. The meander lines may conveniently be formed by slotting respective metal sheets in the pattern illustrated in FIG. 2. Marginal portions 4 of the sheets are left blank for clamping between the side walls 2 and 3 of FIG. 1 and a set of transverse slots 5 is formed in the metal sheet, the slots extending alternately from one or the other marginal portion 4 of a sheet across the center'of the sheet so as to leave between the slots a zigzag conducting path extendshort slots thus forming bars 8 joined together by the connecting links 7.

It should be noted that the Widths of the slots and the relative thickness of the metal sheets compared to their width have been greatly exaggerated in the accompanying drawings so as to show the bars and slots more clearly. In a practical application for use at a wavelength about 6 millimeters, the separation between the side walls of the structure of FIG. 1, or between the marginal portions 4 in FIG. 2, is not quite 0.140 inch, the slots are each 0.0054 inch wide, the bars 8 of the same width, and the slots 5 are 0.0955 inch long, while the metal sheets are made of molybdenum 0.00 25 inch thick, the slots being produced by a photo-etching or spark erosion technique.

Although a travelling wave tube arrangement such as depicted in FIG. 1 has found useful application, the techniques of producing accurately dimensioned electron guns for ribbon-shaped electron beams of precise small sizes are not so highly developed as those for cylindrical electron beams, particularly when it is desired to converge the beam in the electron gun from a cathode of larger area than that of the ultimate beam. Hence for many purposes it is desirable to use a cylindrical electron beam. In order to obtain efficient interaction between an electron beam and the field of a slow wave structure, the beam and the interacting portion of the slow wave structure should be as close as possible without incurring too great an interception of the beam by the structure. For this reason, in place of the planar arrangement of FIG. 1, a cylindrical arrangement such as shown in FIG. 3 is adopted in embodiments of the present invention. Here the metal sheet 1 of FIG. 2 is provided with a curved central zone and a similar metal sheet, oppositely curved, is placed in mirror image relationship with the first sheet so as to provide between them a tunnel for the electron beam. In FIG. 3 two such sheets 10 and 1021 are represented as being clamped together by means of the side walls 2 and 3, the curved portions of the respective metal sheets 10 and 19a forming a cylindrical tunnel about an electron beam 9a.

in the drawing of FIG. 3 the curved portions of the sheets 10 and 10a together form a complete cylinder around the electron beam, the two sheets being clamped one against the other. In some embodiments, however, they may be separated as are the sheets 1 and 1a in FIG. 1, in which case, of course, the curved portions do not each form a semi-cylinder. As is shown in FIG. 3 to either side of the curved portion of each of the metal sheets a flat :margin 11 is left between the curved portion and the adjacent side wall. The margins 11 are distinct from the marginal portions 4, which become, eliectively, part of the side walls. The total distance across each sheet from side wall to side wall measured around the curved part and across the flat parts of a sheet is the same, for the same lower cut-off frequency, as in the arrangement of FIG. 1.

Mention should here be made of the geometrical factors which determine the propagation characteristics of the slow wave structures of FIGS. 1 and 2 and their relationship with other forms of slow wave structure. In general, slow Wave structures exhibit a system of pass bands separated by stop bands. These pass bands are similar to the modes of propagation to be found, say, in waveguides, except that they do not usually overlap as the latter do. If attention be restricted to one pass band only, this can be considered as split up into infinite series of space harmonics, both forward and backward. These space harmonics are not independent; they are all present simultaneously in proportions determined by the geometry of the structure. In an electron tube the direction and velocity of the beam electrons is chosen to interact with the space harmonic of interest. The field of the slow wave structure available, for a given power flow in the structure, to interact with an electron beam may be expressed in terms of an interaction impedance, which is a function of the geometry of the structure.

In the stub supported meander line formed by the sheet 1 or of FIG. 1 and the two side walls, the lower cut off of the first pass band is given by the condition )\=4b, where, as indicated in FIGS. 2, 2b is the width, of the structure between the side walls in FIG. 1, or the trans-, verse distance, in FIG. 2, between the marginal portions 4. The mean length of the transverse portions of the zigzag path between slots in FIG. 2 is indicated as Zeb. If a is less than 0.5, the upper cut off of the first pass band is given by \=2b. For a greater than 0.5 the upper cut off is given by t=4b. In the case where a=0.5 the two limits are identical, the stop band disappears and the first and second pass bands become continuous. In this particular case, at the wavelength of the evanescent stop band the stubs joining the zigzag conducting path of the meander line to the side walls become each M4 long and, electrically speaking, no longer exist.

From the above it is seen that the parameter a infiuences the bandwidth of the structure; it also influences the interaction impedance and further reference will be made below to the manner in which this parameteris to be chosen in embodiments of the invention.

The relations given above also apply when the central zone of the sheet 1 of FIG. 2 is curved out of the plane of the sheet to form one of the sheets 10 or 10a of FIG. 3; hence in FIG. 3 the side walls 2 and 3 are closer together than in FIG. .1.

Referring again to FIGS. 2 and 3, if a be the radius of the cylinder formed by the internal surfaces of the sheets 10 and 10a and if, furthermore, the factor at, determining the length of the slots, is such that Zocb equals am, that is to say, in FIG. 3, if, on the left hand side of the drawing, the slots were to extend around the curved portions of the metal sheets to the junction with the flat portions 11, there would then be obtained a form of stub-supported ring and bar structure. In embodiments of the present invention, however, the dimensions a and ab are not so re lated: rather, for reasons which will be discussed below, the slots in FIG. 3 terminate substantially short of the junction between the curved portions and the flat portions 11 on the left of the drawing or alternatively, they extend a substantial distance beyond that junction on to the flat portion, i.e., the dimension d? is made either substantially greater than or substantially less than m.

In the ring and bar structure, whether of the stub-supported form or not, the distance from ring to ring is uniquely related to the radius of the rings. The pitch, that is from the derivation of the ring and bar structure from a pair of contra-wound helices, the axial. distance from ring center to the next but one ring center, determines the phase shift along the structure from ring to ring, for a given wavelength. In the stub-supported meander line of the present invention the pitch from bar to bar (8 FIG. 2) likewise determines the axial rate of change of phase, other parameters being held constant. On the other hand, the ring and bar structure is equivalent electrically to a pair of similar but contra-wound helices and the performance of a travelling wavetube is determined very largely by the wellsknown factor 'ya where 'y is the radial propagation constant of the slow wave structure. A large value of 'ya results in a low effective interaction impedance. In millimeter wave tubes, for constructional reasons it is important to make the pitch as large as possible. This means, however, that in a ring and bar structure if the pitch is increased, so is the radius of the rings, with the consequence that the factor 'ya becomes high and the interaction impedance low. Thus a compromise has to be made between the conflicting requirements of large pitch and reasonably low value of ya. In the present invention, however, this compromise is no longer necessary. The radius of the curved portion of the meander line may be chosen to provide a reasonable value of 'ya, while the dimensions Zeb may be chosen to suit the pitch from bar to bar independently of the radius a.

The disperson and interaction impedance of the slow wave structure of the present invention is influenced by factors additional to those discussed above. In the illustrated. pattern of FIG. 2 the width of the slot has been made equal to the width of the bars; and the ratio of the thickness of the bars, that is the thickness of the metal sheet 1 of FIG. 2, to the width of the slots is also to be taken into account. If :1 denotes the thickness of the metal sheet and q the slot width, as the ratio d/q increases a greater proportion of the energy of the field of the slow i wave structure is stored between adjacent bars and is relatively inaccessible to the electron beam; thus the interaction impedance of the structure falls. For similar reasons, if q be made small in relation to p, the pitch distance overall from bar to bar, the interaction impedance will fall. A value of the ratio of d/q equal to one half is reasonable and a similar value for the ratio of q/p is satisfactory. The addition of a waveguide ridge between the side walls and connected to them and approaching the stub-supported meander line enables the bandwidth of the meander line to be increased and may be applied in embodiments of the present invention.

In the embodiments of FIGS. 4 and 5 slow wave structures are illustrated diagrammatically with the inclusion of ridge loading as above mentioned. In FIG. 4 two metal sheets 12 and 13, slotted to provide meander lines as in FIG. 2, and having centralzones oppositely curved as in FIG. 3 to form a tunnel for an axial electron beam, are shown sandwiched between side wall portions 20, 2b, 2c, 3a, 3b, and 30 respectively. The two side walls are joined together above and below the slow wave structure and ridges 14 and 15 are brought into juxtaposition with the meander lines. In FIG. 4 each slot 5 extends from one side wall over the curved portion of the structure and terminates short of the junction between the curved portion and the fiat portion 11 of the sheet. In other words, the arcuate length of each of the curved portions of the sheets 12 and 13 (because of the intervening wall members 2b and 3b they are less vthan semi-cylindrical) is less than the dimension 20th of FIG. 2.

In the embodiment of FIG. 5, the arrangement is generally similar to that of FIG. 4 except that here the two sheets 12 and 13 are clamped directly together between the side Wall portions, the portions 2b and 3b of FIG. 4 being omitted. Additionally, the slots 5 here extend substantially beyond the junction of the curved portions and the flat portions of the sheet so that the dimension 2ub substantially exceeds the radius of the cylinder formed by the two curved portions of the metal sheets. This increase in the value of or provides an increase in interaction impedance at the cost of corresponding decrease of bandwidth; the increased interaction impedance is, however, of value in obtaining increased power output from a travelling tube.

In the embodiments described the side walls of the slow wave structure have in each case been straight sided. This is not essential, particularly when one considers the effect of ridge loading as in FIGS. 4 and 5. Thus it would be quite feasible to clamp the meander line sheets of the invention across the major diameter of a metal tube of elliptical cross section split into two portions about its major axis and in which the portions adjacent its minor axis corresponded to the ridges 14 and 15 of FIGS. 4 and 5.

What is claimed is:

1. In a travelling wave tube having a cylindrical electron beam and a slow wave structure secured to metal side walls, the beam-interacting portions thereof conforming closely to the profile of the beam and providing a tunnel therefor, the said beam-interacting portions of the slow wave structure include a pair of similarly slotted but oppositely curved metal sheets having flat longitudinally extending marginal portions on each side fixed to said side walls and wherein each sheet is slotted to form with the side walls a stub-supported meander line providing a zigzag conducting path from end to end of the slow wave structure, each sheet having a first set of transverse slots extending alternately from each side Wall and terminating within the flat portion adjacent said side wall spaced from the curved portion and a second set of transverse slots along the length of said structure extending alternately with said first set from each side wall and flat portion over the curved portion of the sheet beyond the axis of the tunnel and terminating at a position spaced from the junction of the curved portion of the sheet with the opposite flat marginal portion, said second set of slots being aligned with said first set extending from the opposite walls and spaced therefrom by a portion of said metal sheet.

2. The travelling wave tube slow wave structure of claim 1 wherein said pair of slotted sheets are clamped across the space between the two side Walls and said second set of slots extend alternately from each side wall transversely across the metal sheet to a distance beyond the junction of the curved and opposite flat marginal portions.

wherein the two side walls are mechanically connected together to provide an enclosure about the said sheets, and a pair of opposed metal ridges project between the side walls from the connected portions of said side walls towards the curved portions of the said metal sheets.

4. The travelling wave tube slow wave structure of claim 1 wherein said pair of slotted sheets are clamped between the two walls and said second set of slots extend alternately from each side wall transversely across the metal sheet to a distance short of the junction of the curved and opposite flat marginal portions.

References Cited UNITED STATES PATENTS 2,885,592 5/1959 Robinson et a1. 315-393 X 2,936,396 5/1960 Currie 315-39.3 X 2,999,959 9/1961 Kluver 315--39.3 3,043,984 7/1962 Stephenson 3153.5 3,069,588 12/1962 Skowron et al. 333--31 3,076,909 2/1963 Hogg et al. 315-3.6 X 3,181,090 4/1965 Ash 333-31 ELI LIEBERMAN, Primary Examiner,

3. In a travelling wave tube as claimed in claim 1 S- H MQ -a a e 

1. IN A TRAVELLING WAVE TUBE HAVING A CYLINDRICAL ELECTRON BEAM AND A SLOW WAVE STRUCTURE SECURED TO METAL SIDE WALLS, THE BEAM-INTERACTING PORTIONS THEREOF COMFORMING CLOSELY TO THE PROFILE OF THE BEAM AND PROVIDING A TUNNEL THEREFOR, THE SAID BEAM-INTERACTING PORTIONS OF THE SLOW WAVE STRUCTURE INCLUDE A PAIR OF SIMILARLY SLOTTED BUT OPPOSITELY CURVED METAL SHEETS HAVING FLAT LONGITUDINALLY EXTENDING MARGINAL PORTIONS ON EACH SIDE FIXED TO SAID SIDE WALLS AND WHEREIN EACH SHEET IS SLOTTED TO FORM WITH THE SIDE WALLS A STUB-SUPPORTED MEANDER LINE PROVIDING A ZIGZAG CONDUCTING PATH FROM END TO END OF THE SLOW WAVE STRUCTURE, EACH SHEET HAVING A FIRST SET TRANSVERSE SLOTS EXTENDING ALTERNATELY FROM EACH SAID WALL AND TERMINATING WITHIN THE FLAT PORTION ADJACENT SAID SIDE WALL SPACED FROM THE CURVED PORTION AND A SECOND SET OF TRANSVERSE SLOTS ALONG THE LENGTH OF SAID STRUCTURE EXTENDING ALTERNATELY WITH SAID FIRST SET FROM EACH SIDE WALL AND FLAT PORTION OVER THE CURVED PORTION OF THE SHEET BEYOND THE AXIS OF THE TUNNEL AND TERMINATING AT A POSITION SPACED FROM THE JUNCTION OF THE CURVED PORTION OF THE SHEET WITH THE OPPOSITE FLAT MARGINAL PORTION, SAID SECOND SET OF SLOTS 